Investigating the device that you have paid for is obviously legal.
Despite of whether it is a car, a PC or a metal hammer.
I have a right to burn my car, crash and smash it to floatsam and jetsam.
Why i cannot look what is inside the "black box" of silver ECU

If it is illegal then how call the fact of selling a couple of bites through the j1962 OBD-II socket for the 1000$ ?

And they ask a payment from every reflashed ECU!

May be they want all the cars to pay them for the atmosphere air that feed our engines?

The DMCA has clear cutouts for reverse engineering to ensure compatibility and interoperability. Running code from an ECU you own on a modified engine you own which requires modifications clearly falls under the interoperability exception in the DMCA.